Semiconductor light trap devices

ABSTRACT

Embodiments relate to buried structures for silicon devices which can alter light paths and thereby form light traps. Embodiments of the lights traps can couple more light to a photosensitive surface of the device, rather than reflecting the light or absorbing it more deeply within the device, which can increase efficiency, improve device timing and provide other advantages appreciated by those skilled in the art.

TECHNICAL FIELD

The invention relates generally to silicon devices and more particularlyto optoelectronic silicon devices.

BACKGROUND

In optoelectronic devices, light rays are absorbed and generate chargecarriers within the device. These charge carriers typically are desiredto be generated within a particular light ray absorption region, whichcan be defined by a depth within the device, such that they can becollected near the surface of the device.

Charge carriers generated deeper than the depth of this region can bethought of us undesirable noise. Conventional approaches to dealing withthese charge carriers often relate to transporting them to the surfaceby extended electric fields or annihilating them by the targetedintroduction of recombination centers. The former is not suitable in allsituations, such as those with regions that must remain free ofelectrical fields for physical reasons, and is also limited by availablevoltage, while the latter reduces internal quantum efficiency and can betechnically difficult to realize at very high doping atom densities.Reduced quantum efficiency in turn can affect devices dimensions, andincreased complexity and technological challenges can increase costs,which are undesired.

A variety of optical sensors use infrared light for recognition ofobjects and distances. It is known that with increasing wavelength, thedepth of penetration of light into a silicon structure increases. Withthe increasing depth of penetration, for time critical applications, thedelay of slow diffusion currents from the greater depth to which theinfrared light penetrates to the surface of the devices is slow. Thiscan create time delays that become significant in view of the greatertime required for the slow diffusion currents to reach the surface. Toaddress these issues, for infrared wavelengths in the range of 900 nm,the prior art commonly uses electrical confinement such as built infields. However, these electrical confinement efforts can become costlyand can increase production costs to a level that is economicallyunfeasible.

In a wide range of applications, infrared light is the light signal ofchoice because it is invisible to human beings. This permitsmeasurements to be made without the awareness of human observers. In theprior art, such as proximity sensors or time of flight sensors, lossesof quantum efficiency are accepted in most cases. For time criticalapplications, losses in quantum efficiency are sometimes even increasedby implementation of recombinative regions in deeper areas of thesilicon structure in an effort to eliminate disturbing backgroundcurrent that arises from slowly diffusing charge carriers. A graphdepicting the increased penetration depth with longer wavelength isdepicted in FIG. 7.

In the field of photovoltaics, in the prior art, a variety of so-called“light trapping” techniques have been disclosed. In the photovoltaicprior art, a variety of structures have been investigated for improvingthe light performance including inverted pyramids, dice-grooving ofmulti-crystalline material or perpendicularly oriented grooves on thefront and the back surface of the photovoltaic device. However, thesetechniques are unsuitable for use in CMOS integrated circuitapplications because of the structures themselves. An effectivephotovoltais uses a front and a rear surface that directly covers theactive absorption region of the optical electronic silicon device. Instandard CMOS technologies, the rear side of the structure is defined byback thinning in back end processing. Accordingly, dedicated structuresfor light trapping cannot be defined during a front end fabricationsequence.

Many optical electronic silicon devices benefit from having a smallpixel size. High internal quantum efficiency is important to achievingsmall pixels sizes which require small device dimensions. In particular,in time critical applications, it is important to define a smallabsorption region to reduce noise components that are produced by chargecarriers that are generated deep in the volume of the semi-conductor. Asdiscussed above, this can be difficult because silicon as an indirectsemi-conductor absorbs infrared wavelength spectral light componentsonly weakly. Because of this, the penetration depth for light atwavelengths of between 800 and 900 nm, is approximately 12 micrometersto 32 micrometers. In situations where it is beneficial for photogenerated charge carriers to be collected near the surface of thesemi-conductor, this property of the material can be problematic fortime critical device operation. According to the prior art, to addressthis problem, charge carriers that are generated deep in the volume ofthe silicon semi-conductor have been transported to the surface by theapplication of extended electric fields. These electric fields are builtup in lightly doped zones and are limited by the available voltage.Accordingly to the prior art, regions of the semi-conductor that need toremain free of a field for physical reasons are designed such that theelectron hold pairs generated therein were annihilated by the targetedintroduction of recombination centers. This approach leads to reducedquantum efficiency and is technologically difficult to implement at veryhigh impurity atom densities. Under the circumstances, the recombinationactive zone is overgrown with a lightly doped epitaxial layer, which isreadily susceptible to being dislocated on such a support.

SUMMARY OF THE INVENTION

Embodiments relate to light trap devices, systems and methods.

In an embodiment, a device comprises a silicon structure having anexposed surface to be exposed to light rays; a photoelectrically activeportion located beneath the exposed surface within the siliconstructure; and an internal structure located beneath the active portion,the internal structure having an interface surface generally parallel tothe exposed surface at which total internal reflection of light raysoccurs such that at least some the light rays are redirected toward thephotoelectrically active portion.

In an embodiment, a device comprises a silicon structure having anexposed surface to be exposed to light rays, the surface having aexposed surface plane, the exposed surface defining refractive facetsthat are angled to be non-parallel to the surface plane; aphotoelectrically active portion located beneath the surface within thesilicon structure; and an internal structure located beneath the activeportion, the internal structure having an interface surface, at leastpart of the interface surface being generally parallel to the exposedsurface plane and at which total internal reflection of light raysoccurs such that at least some of the light rays are redirected towardthe photoelectrically active portion.

In an embodiment, a device comprises a silicon structure having anexposed surface to be exposed to light rays; a photoelectrically activeportion located beneath the surface within the silicon structure; aninternal structure located beneath the photoelectrically active portion,the internal structure having at least one interface surface notparallel to the exposed surface at which total internal reflection oflight rays occurs such that at least some light rays are redirectedtoward the photoelectrically active portion, toward the exposed surfaceor both; and a perimeter structure located proximate a perimeter of thesilicone structure and extending at least partially above the internalstructure and below the exposed surface, the perimeter having aninternally directed interface at which total internal reflection of atleast some of the light rays occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a side cross-sectional view of a light trap structure in asilicon structure according to an embodiment.

FIG. 2 is a side cross-sectional view of a light trap structure in asilicon structure according to an embodiment.

FIG. 3 is a side cross-sectional view of a light trap structure in asilicon structure according to an embodiment.

FIG. 4 is a side cross-sectional view of a light ray striking a siliconstructure having a light trap structure according to an embodiment.

FIG. 5A is a side cross-sectional view of a silicon structure accordingto an embodiment.

FIG. 5B is a side cross-sectional view of a plurality of trenches formedin a silicon structure according to an embodiment.

FIG. 5C is a side cross-sectional view of a plurality of cavities formedfrom the trenches of FIG. 5B according to an embodiment.

FIG. 5D is a side cross-sectional view of a photodetector devicecomprising a light trap structure according to an embodiment.

FIG. 6 is a side cross-sectional view of a light trap structure in asilicon structure having a light-focusing roof structure according to anembodiment.

FIG. 7 is a graph depicting the penetration depth of light as a functionof wavelength into silicon;

FIG. 8 is a side cross sectional and perspective view of a siliconstructure according to an embodiment;

FIG. 9 is a side cross sectional and perspective view of a siliconstructure according to an embodiment;

FIG. 10 is a side cross sectional view of a silicon structure includinga layer of adjacent tubes each having a long axis;

FIG. 11 is a graph depicting photo current as a function of gatevoltage;

FIG. 12 is a graph depicting power as a function of trench widthsaccording to an embodiment; and

FIG. 13 is a side cross sectional view of a silicon structure includinga perimeter trench structure and an internal structure and apolycrystalline silicon liner.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Embodiments relate to buried structures for silicon devices which canalter light paths and thereby form light traps. Embodiments of thelights traps can couple more light to a photosensitive surface of thedevice, rather than reflecting the light or absorbing it more deeplywithin the device, which can increase efficiency, improve device timingand provide other advantages appreciated by those skilled in the art.

In an embodiment, a device comprises a silicon structure having asurface to be exposed to light rays; and a light trap structure formedwithin the silicon structure spaced apart from the surface andcomprising a plurality of light trap elements adjacent one another andeach having a surface proximate to and unparallel with the surface to beexposed to light rays.

In an embodiment, a light trap structure buried within a siliconstructure and comprises a plurality of light trap elements adjacent oneanother and each having a surface proximate to and unparallel with asurface of the silicon structure to be exposed to light rays.

In an embodiment, a method comprises providing a silicon structure; andforming a plurality of light trap elements adjacent one another in thesilicon structure and each having a surface proximate to and unparallelwith a surface of the silicon structure to be exposed to light rays.

One concern that arises with the utilization of light traps, asdescribed herein, is that photo generation of free charge carriers isconcentrated in a thin layer, having a thickness of a few micrometers,at the surface. The use of non-plane-parallel structural elements toproduce total internal reflection of light at interfaces of thestructures directs light rays such that they may travel in paths thatare nearly parallel to the chip surface. This can lead to light raysstriking interfaces between pixels at angles below the critical anglefor total internal reflection and thus lead to optical cross talk withadjacent pixels.

An electronic silicon device defines a trench structure that is locatednear the perimeter of a pixel. A trench is located at or near the pixelperimeter and may be lined with a polycrystalline silicon liner withwhich electrical contact can be made and which, according to anembodiment, is overgrown with a thermal oxide. The perimeter trench maybe encapsulated on a top side thereof, for example, by use of an HDPoxide deposition process. The trench performs both optical andelectrical functions. Because of the polycrystalline silicon liner withwhich contact can be made, the trench can influence the recombinationbehavior at a trench wall. Depending on applied voltage and accumulationlayer or an inversion layer can be produced proximate the trench wall.If the photosensitive element is to be laterally passivated, thepresence of the accumulation layer provides for a reduced surfacerecombination rate.

An optical effect of the trench is the suppression of optical cross talkbetween adjacent pixels. Almost all incident light rays are totallyinternally reflected upon encountering the trench cavity interface.Transition from an optical medium having a quite high refractive indexto a medium having a refractive index approximating one leads to a smallcritical angle of approximately sixteen degrees for total internalreflection. Accordingly, radiation components that are traveling nearlyparallel with the front surface of the electronic device are retainedwithin the pixel and do not affect adjacent pixels. The width of the airgap of the trench affects the efficiency of elimination of optical crosstalk. Small gap widths may permit the passage of a certain amount oflight. According to an embodiment, the gap width may even be in therange of 1 to 3 times the wavelength of the incident light.

In an embodiment, an optical electronic structure includes a perimetertrench that provides electrical and optical effects. The trenchstructure may include an electrically conductive liner and a cavitywithin. This results in an adjustable electrical recombination activityat the interface and a small critical angle for total internalreflection of incident radiation.

According to an embodiment, a photolectronic device includes pyramidalfacets formed thereon or therein. According to another embodiment, in acase of masked structural process, a photoelectronic device includesinverted pyramid or roof structures thereon or therein. Roof structuresmay have a long axis that is parallel in several roof structures.

In an another embodiment, a photoelectronic device includes a roofstructure that refractively redirects light and a deeper cavitystructure having an interface that causes total internal refraction oflight back toward the roof structure.

According to another embodiment, a photoelectronic structure includes aplurality of light trap elements having a surface non-parallel with thesurface to be exposed to light rays that redirect incoming light bytotal internal reflection and a more deeply located internal structurehaving an interface that redirects light upward by total internalreflection.

According to an embodiment, internal structures may be arranged so thata roof structure has many optical facets having a long axis in a firstdirection and a more deeply located tubular structure having manyelongate tubes is located more deeply in the photoelectronic device andis orthogonal to or otherwise non-parallel to the long axis of the roofstructure.

According to an embodiment, a silicon structure has an exposed surfaceto be exposed to light rays; a photoelectrically active portion locatedbeneath the exposed surface within the silicon structure; and aninternal structure located beneath the active portion, the internalstructure having an interface surface generally parallel to the exposedsurface at which total internal reflection of light rays occurs suchthat at least some the light rays are redirected toward thephotoelectrically active portion.

According to an embodiment, a silicon structure has an exposed surfaceto be exposed to light rays, the surface having a exposed surface plane,the exposed surface defining refractive facets that are angled to benon-parallel to the surface plane; a photoelectrically active portionlocated beneath the surface within the silicon structure; and aninternal structure located beneath the active portion, the internalstructure having an interface surface, at least part of the interfacesurface being generally parallel to the exposed surface plane and atwhich total internal reflection of light rays occurs such that at leastsome of the light rays are redirected toward the photoelectricallyactive portion.

According to an embodiment, a silicon structure has an exposed surfaceto be exposed to light rays; a photoelectrically active portion locatedbeneath the surface within the silicon structure; an internal structurelocated beneath the photoelectrically active portion, the internalstructure having at least one interface surface not parallel to theexposed surface at which total internal reflection of light rays occurssuch that at least some light rays are redirected toward thephotoelectrically active portion, toward the exposed surface or both;and a perimeter structure located proximate a perimeter of the siliconestructure and extending at least partially above the internal structureand below the exposed surface, the perimeter having an internallydirected interface at which total internal reflection of at least someof the light rays occurs.

Referring to FIG. 1, an embodiment of a light trap structure 100embedded in a silicon structure 102 is depicted. Silicon structure 102can comprise or consist of silicon in various embodiments, and in stillother embodiments can comprise material(s) other than silicon. Lighttrap structure 100 comprises a plurality of light trap elements 104arranged immediately adjacent one another. Light trap elements 104 canhave length and/or width dimensions of about 0.5 μm to about 3 μm,height dimensions of about 1 μm to about 5 μm, and be disposed about 0.7μm to about 50 μm from a top surface of silicon structure 102 inembodiments, though these dimensions can vary in other embodiments. Forexample, light trap elements 104 can be formed from trenches that extendpartially, substantially or entirely along a length of silicon structure102.

In embodiments, light trap elements 104 comprise cavities filled withair or some other suitable gas or material. Light trap structure 100also can comprise a single light trap element 104 having the overalloutline shape of structure 100 depicted in FIG. 1 but with theindividual cavities being linked or joined to form a single cavitystructure. Light trap structure 100 can comprise a single row of lighttrap elements 104 or a matrix of light trap elements, for examplecomprising four rows having eight elements 104 each. Though depicted inFIG. 1 as extending from one side of silicon structure 102 to the other,in other embodiments light trap structure can be formed in only aportion of silicon structure 102, such as is depicted in FIG. 2.

In the embodiment of FIG. 1, light trap elements 104 comprise curved,rounded or arched top and bottom (with respect to the depiction of thedrawing on the page) portions. In other embodiments, only the top can becurved and the bottom can have some other shape or structure, forexample being generally flat and therefore parallel with a top surfaceof silicon structure 102. In still other embodiments, light trapelements 104 can have other shapes, such as peaked or pointed asdepicted in FIG. 3. In general, however, it can be seen that the surfaceof each light trap element 104 adjacent or opposing the upper surface ofsilicon structure 102 are angled, curved or otherwise unparallel withthat upper surface.

The particular radius of curvature (FIGS. 1 and 2) or angle (FIG. 3) ofthe surfaces of light trap elements 104 can vary in embodimentsaccording to materials used, light wavelengths and other factors. Forexample, silicon has a relatively high refractive index, such as betweenabout 3.5 and 5.5 depending upon the wavelength of light. For a lightwavelength of about 850 nm, a critical angle of about 16 degrees exists:all light rays impinging from silicon structure 102 on an interface witha vacuum or air layer at an angle greater than about 16 degrees will betotally reflected. Thus, light rays which enter silicon structure 102and are reflected internally at an angle greater than 16 degrees will besubject to total reflection within silicon structure 102. In otherembodiments, silicon structure 102 can comprise some other material,such as germanium, gallium arsenide, another 3-5 group semiconductormaterial, or another suitable material. Germanium and gallium arsenide,for example, generally have high refraction indices, like silicon, suchthat the total reflection angle will be below about 20 degrees in theseembodiments as well, though one skilled in the art will appreciate thatthe angle can vary according to the properties of the material(s) used.

Referring to FIG. 4, a light ray 106 impinging on silicon structure 102is reflected by light trap structure 100 back toward the surface 108,which is a silicon-air interface. Because of the arched structure of thelight trap elements 104, ray 106 impinges on surface 108 from withinsilicon structure 102 at an angle α greater than about 16 degrees. Thiscauses ray 106 to be reflected back toward light trap structure 100,which again causes ray 106 to strike surface 108 at an angle greaterthan about 16 degrees and be reflected internally. This internalreflection caused by altering the path of light ray 106 by light trapstructure 100 can improve the efficiency of the device because a largerportion of light impinging on the device can be used by the device, suchas by coupling more light to a photo surface of the device rather thanbe reflected externally.

The embodiment of FIG. 3 also can provide advantages. For example, thepeaked light trap elements 104 of FIG. 3 can be more efficient bypresenting a smaller portion of each element 104 which would cause thelight to be reflected externally. In other words, the number of lightrays striking the very peak of the light elements 104 generally will bevery small, such that more light rays will be reflected internallysimilar to as depicted in FIG. 4.

Referring next to FIGS. 5A-5D, one example process for forming lighttrap structure 100 is depicted. In one embodiment, a Venezia process forforming cavities in monocrystalline silicon can be used, though otherprocesses can be used in this and other embodiments. In FIG. 5A, asilicon structure 102, such as a silicon wafer, is provided. In FIG. 5B,at least one trench 110 is etched in silicon structure 102. Siliconstructure 102 is then exposed to a hydrogen atmosphere and a reflowprocess or epitaxy step, which results in the silicon-on-nothingstructure 102 depicted in FIG. 5C, in which light trap elements 104 havebeen formed from each of the at least one trenches 110. FIG. 5D depictsone example of a final photo-sensitive structure 112, such as a photodiode, after formation of a photo detector 114 between light trapstructure 100 and surface 108. As depicted in FIG. 5D, photo-sensitivestructure 112 forms only a portion of surface 108, such as aphoto-sensitive “window” of the device, though this can vary in otherembodiments to include an entire surface or a plurality ofphoto-sensitive structures spaced apart at a surface or some otherarrangement.

The process can vary for other embodiments, such as the peaked lighttrap structure 100 of FIG. 3. For such an embodiment, for example,silicon structure 102 and trenches 110 can be treated with an alkalinesolution to anisotropically etch the silicon, thereby uncovering the(111) surface of the silicon, which results in the peaks of FIG. 3 asthe angle of about 54.7 degrees between (100) and (111) planes of thesilicon of silicon structure 102. Other surfaces of the silicon, such asthe (110) surface also can be used in embodiments to produce differentangles of the peaks, which vary with the crystal orientation of thesilicon, though other angles can be less efficient. For example, theangle can be between about 50 degrees and about 60 degrees, such asbetween about 53 and 56 degrees, for example about 54.7 degrees inembodiments.

Other features and elements can be incorporated in embodiments. Forexample, and referring to FIG. 6, a roof structure 116 can be used withlight trap structure 100 to further maximize quantum efficiency. Such aroof structure 116 can be used in photovoltaics, such as high-poweredsolar cells, and other applications to better channel or focus lightrays to photosensitive elements below. In embodiments, roof structure116 comprises a plurality of inverted pyramid, funnel or otherlight-focusing structures and can be formed by a masked potassiumhydroxide solution etch or some other suitable process step(s). Roofstructure 116 need not be a separate layer as depicted but instead canbe an area or portion of silicon structure 102 that is etched, patternedor otherwise altered to include one or more light-focusing structures.In another example, light trap structure 100 can comprise a thermalliner oxide layer in order to electrically passivate the interfacebetween surface 108 and/or to electrically insulate the region from thesubstrate below. Using roof structure 116 and/or a thermal liner oxidelayer in combination with a light trap structure can thereby providebenefits in embodiments.

Referring to FIG. 7, a graph relating penetration depth as a function ofwavelength of light into a silicon structure is depicted. As can beseen, penetration depth increases as a function of wavelength.Accordingly, infrared light passes more deeply into the siliconstructure before it is absorbed than shorter wavelengths of light. Sinceinfrared light is preferred for many functions because of itsinvisibility to human vision, the present invention redirects infraredlight into the photoelectrically active part of a silicon substrate,thus increasing efficiency of each pixel.

Referring to FIG. 8, an embodiment of a silicon structure 150 isdepicted. According to the depicted embodiments, silicon structure 150includes exposed surface 152, photoelectrically active portion 154,internal structure 156, interface surface 158 and refractive facets 160.Internal structure 156 may be formed as a cavity generated byapplication of a silicon on nothing processing scheme. Refractive facets160 of expose surface 152 may be structured by anisotropic etching witha strong alkaline substance, for example, KOH or TMAH. Accordingly,refractive facets 160 may be formed as pyramidal facets or, byapplication of a masked structuring process inverted pyramid or roofstructures. In the depicted embodiment, refractive facets 160 refractlight ray 162 directing it at an angle to exposed surface 152 towardinterface surface 158 where it is totally internally reflected atinterface surface 158 and redirected toward refractive facets 160 whereit is refracted back outwardly from exposed surface 152. Despite thesimply configuration of the embodiment depicted in FIG. 8, the lightpath is extended by a factor of approximately 2.5 if silicon oxide isused as filler in roof structure 164 of refractive facets 160 at exposedsurface 152. If inverted pyramids are used, the light path extensionfactor increases beyond 2.5.

Referring to FIG. 9, another depicted embodiment includes scatteringfacets 168 at interface surface 158 between silicon structure 150 andscattering facet cavities 170 and presents exposed planar surface 172.Internal structure 156 presenting interface surface 158 is generallysimilar to that described above. Scattering facet cavities 170 may belocated above or within photoelectrically active portion 154. Scatteringfacets 168 are oriented non parallel to exposed planar surface 172. Asdepicted in FIG. 9, light ray 174 passes through exposed planar surface172 and is totally internally reflected at scattering facets 168. Totalinternal reflection may also occur at interface surface 158 and exposedplanar surface 172 when light ray 174 strikes the surfaces at an anglegreater than the critical angle of total internal reflection forsilicon. As discussed above, the critical angle is approximately 16°.Accordingly, light ray 174 may incur multiple total internal reflectionsat scattering facets 168, the underside of exposed planar surface 172and at interface surface 158.

Accordingly, light ray 174 passes through photoelectrically activeportion 154 multiple times thus creating further photoelectricalactivity until the energy of light ray 174 is absorbed or until itsstrikes an internal surface of silicon structure 166 at an angle lessthan the critical angle and leaves silicon structure 166.

Referring to FIG. 10, another embodiment is depicted. Silicon structure166, according to the depicted embodiment includes another configurationof internal structure 156. Internal structure 156 may be formed as layer176 of adjacent tubes 178. Adjacent tubes 178 have long axis 180 whichis perpendicular to the plane of the drawing of the depicted embodiment.Internal structure 156 including adjacent tubes 178 may be utilized incombination with roof structure 164 as depicted in FIG. 8. As discussedabove, long axis 180 of adjacent tubes 178 is perpendicular to the pagein FIG. 10. When utilized with roof structure 174, according to anexample embodiment, roof structure 164 has long axis 182 depicted inFIG. 10. Accordingly, long axis 180 of adjacent tubes 178 and long axis182 of roof structure 164 may be oriented orthogonal to one another orotherwise non-parallel to one another.

Referring particularly to FIG. 13, according to the depicted embodiment,silicon structure 184 includes exposed surface 186, photoelectricallyactive portion 188, perimeter structure 190 and internal structure 192.

Perimeter structure 190, according to the depicted embodiment, islocated generally adjacent perimeter 194 of silicone structure 184.Accordingly, perimeter structure 190 surrounds photoelectrically activeportion 188. In addition, internal structure 192 borders a bottom ofphotoelectically active portion 188. Perimeter structure 190 may presenttrench structure 196. Accordingly, in the case of a pixel, trenchstructure 196 delimits perimeter 194 of the pixel.

According to an embodiment, trench structure 196 further includespolycrystalline silicon liner 198 and may include an encapsulation 200at a top thereof. Polycrystalline silicon liner 198 is structured sothat electrical contact can be made therewith. According to anembodiment, polycrystalline silicon liner 198 may be overgrown with athermal oxide. Encapsulation 200 is located at a top of trench structure196 and may be formed for example, by application of an HDP oxidedeposition process.

According to an embodiment, trench structure 196 performs at least twofunctions. First, because of polycrystalline silicone liner 198 withwhich contact can be made, it is possible to influence recombinationbehavior at trench interface 202. Depending on applied voltage, anaccumulation layer or an inversion layer can be produced. If siliconstructure 150 is to be laterally passivated, the accumulation layerprovides for a reduced surface recombination rate. Referring to FIG. 11,the depicted graph illustrates this effect. The accumulation of majoritycharged carriers leads to a higher photocurrent at a voltage of minus 2volts. Given a positive voltage at the trench electrode and a p-typedoped bulk, an inversion layer is established. In this case, aconnection of the inversion layer and a n-type doped emitter leads toincreased photoefficiency, too. Here, the inversion layer becomes partof the emitter and electron-hole pairs are separated before reaching therecombinative trench surface.

A second function of perimeter structure 190 and trench structure 196 issuppression of optical cross talk between adjacent pixels, because oftotal internal reflection at trench interface 202. As is known, thetransition at interface from an optically dense medium, here siliconhaving a refractive index of approximately 3.6 to a less optically densemedium having a refractive index approximately equal to one leads to asmall critical angle of approximately 16° from a normal to a surface fortotal internal reflection. Even light ray 204 components redirected byinternal structure 192 relatively close to parallel to exposed surface186 are held within silicon structure 184 and inhibited from passinginto adjacent pixels.

Referring particularly to FIG. 12, it should be understood that opticaltunneling may occur in the case of small width 206 of trench structure196. As can be seen in the graph depicted in FIG. 12, the amount oftransmitted radiation is an inverse function of light wavelength and aninverse function of width 206. The graph in FIG. 12, depicts anexperimentally determined relationship related to trench structure 196.When a width 206 of trench structure 196 of approximately one wavelengthis utilized light transmission is damped by approximately two orders ofmagnitude.

Accordingly, according to one embodiment, width 206 is greater thanapproximately one wavelength of the light desired to be totallyinternally reflected. According to another embodiment, width 206 isgreater than two wavelengths of the light to be totally internallyreflected and according to another embodiment, width 206 is greater thanapproximately three wavelengths of the light to be totally internalreflected.

Referring to FIG. 13, internal structure 192 includes reflective facets208. Reflective facets 208 are angled similarly to other structuresdiscussed elsewhere in this application. At reflective facets 208, totalinternal reflection occurs which causes a least some of light rays 204to strike trench interface 202 at an angle greater than the criticalangle for total internal reflection and some of light rays 204 to strikeexposed surface 186 at an angle greater than the critical angle. Thus,the depicted embodiment largely inhibits optical and electricalinteraction between adjacent pixels. The maintenance of light rayswithin each pixel leads to greater absorption of light energy andgreater efficiency.

Embodiments thereby provide improved light trap structures for silicondevices which can alter light paths. Embodiments of the lights traps cancouple more light to a photosensitive surface of the device, rather thanreflecting the light or absorbing it more deeply within the device,which can increase efficiency, improve device timing and provide otheradvantages appreciated by those skilled in the art.

Various embodiments of systems, devices and methods have been describedherein. These embodiments are given only by way of example and are notintended to limit the scope of the invention. It should be appreciated,moreover, that the various features of the embodiments that have beendescribed may be combined in various ways to produce numerous additionalembodiments. Moreover, while various materials, dimensions, shapes,configurations and locations, etc. have been described for use withdisclosed embodiments, others besides those disclosed may be utilizedwithout exceeding the scope of the invention.

Persons of ordinary skill in the relevant arts will recognize that theinvention may comprise fewer features than illustrated in any individualembodiment described above. The embodiments described herein are notmeant to be an exhaustive presentation of the ways in which the variousfeatures of the invention may be combined. Accordingly, the embodimentsare not mutually exclusive combinations of features; rather, theinvention can comprise a combination of different individual featuresselected from different individual embodiments, as understood by personsof ordinary skill in the art. Moreover, elements described with respectto one embodiment can be implemented in other embodiments even when notdescribed in such embodiments unless otherwise noted. Although adependent claim may refer in the claims to a specific combination withone or more other claims, other embodiments can also include acombination of the dependent claim with the subject matter of each otherdependent claim or a combination of one or more features with otherdependent or independent claims. Such combinations are proposed hereinunless it is stated that a specific combination is not intended.Furthermore, it is intended also to include features of a claim in anyother independent claim even if this claim is not directly madedependent to the independent claim.

Any incorporation by reference of documents above is limited such thatno subject matter is incorporated that is contrary to the explicitdisclosure herein. Any incorporation by reference of documents above isfurther limited such that no claims included in the documents areincorporated by reference herein. Any incorporation by reference ofdocuments above is yet further limited such that any definitionsprovided in the documents are not incorporated by reference hereinunless expressly included herein.

For purposes of interpreting the claims for the present invention, it isexpressly intended that the provisions of Section 112, sixth paragraphof 35 U.S.C. are not to be invoked unless the specific terms “means for”or “step for” are recited in a claim.

The invention claimed is:
 1. A device comprising: a silicon structurehaving an exposed surface to be exposed to light rays; aphotoelectrically active portion located beneath the surface within thesilicon structure; an internal structure located beneath thephotoelectrically active portion, the internal structure having at leastone interface surface not parallel to the exposed surface at which totalinternal reflection of light rays occurs such that at least some lightrays are redirected toward the photoelectrically active portion, towardthe exposed surface or both; and a perimeter structure located proximatea perimeter of the silicone structure and extending at least partiallyabove the internal structure and below the exposed surface, theperimeter having an internally directed interface at which totalinternal reflection of at least some of the light rays occurs.
 2. Thedevice of claim 1, wherein the perimeter structure is formed as aperimeter trench.
 3. The device of claim 2, wherein the perimeter trenchhas a width greater than approximately one wavelength of the light to betotally internally reflected and less than approximately fourwavelengths of the light to be totally internally reflected.
 4. Thedevice of claim 1, wherein the perimeter trench further comprises aconducting liner.
 5. The device of claim 1, wherein the perimeter trenchfurther comprises an encapsulation at a top thereof.
 6. The device ofclaim 1, wherein the internal structure comprises a cavity defined byreflective facets oriented non parallel to the exposed surface.
 7. Thedevice of claim 1, wherein the conducting liner comprises a polysiliconliner.